Vibrational spectroscopy can be a powerful tool for chemical composition analysis. The interaction of light and matter can allow the interrogation and identification of vibrational states that are unique to each molecular structure. In this regard, Raman vibrational spectroscopy can provide exceptional molecular identification. Based on inelastic scattering of photons off molecular vibrations, the Raman effect can be very well suited for chemical recognition.
FIGS. 1 and 2 depict energy diagrams that illustrate a conventional Raman scattering process. If a probe beam with energy 155 is incident on a sample with ground state 100 (designated as |c>), available excited vibrational mode 110 (designated as |b>), and available energy level 150 (designated as |a>) as depicted in FIGS. 1 and 2, then the sample can generate a spontaneous Raman spectrum—one associated transition of which is depicted with energy 135, and which corresponds to a “Stokes shift” in the energy 155 of the probe beam. Note that a virtual state 130 (between excited vibrational mode 110 and energy level 150) is identified as bridging the transition between the probe beam of energy 155 and the emitted radiation associated with the Stokes shift at energy 135. The excited vibrational mode correlated with the Stokes shift is a vibrational mode that is randomly excited (a difference between levels |b>, and |c>). Furthermore, although excited vibrational mode 110 is identified in FIGS. 1 and 2 with a single label |b>, there can be many vibrational modes that are available and that differ in energy (which is indicated by the double line in FIGS. 1 and 2).
A disadvantage of conventional Raman spectroscopy, which relies upon incoherent scattering, is the low probability of this process. For example, a low conversion efficiency of pump photons into Stokes shifted photons can make spontaneous Raman measurements sometimes lengthy and undesirable for fast identification of molecular vibrations. Where applied to the nondestructive detection of trace explosives from large distance in the presence of interferants, it can be difficult to use conventional Raman spectroscopy in a rapid and reliable manner.